Characterization of a synthetic peptide impurity by fast-atom bombardment-tandem mass spectrometry and gas chromatography-mass spectrometry.

Fast-atom bombardment-tandem mass spectrometry (FAB-MS-MS) was used to characterize the structure of atosiban, a synthetic oxytocin antagonist, and one of its synthesis-related impurities. The nature of the structural modification in the impurity of interest, replacement of the proline residue by 5-aminovaleric acid, was determined directly from its product ion MS-MS spectrum. Confirmation of the identity of the impurity was accomplished with GC-MS.